MERGERS & ACQUISITIONS (Details 2) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
2012 Acquisitions
Unaudited pro forma supplemental information
Pro forma net interest income	$ 157,343,000	$ 146,682,000
Pro forma non-interest income	67,437,000	64,170,000
Pro forma net income from continuing operations	33,442,000	21,251,000
Pro forma earnings per share from continuing operations:
Basic (in dollars per share)	$ 1.36	$ 0.99
Diluted (in dollars per share)	$ 1.35	$ 0.98
Beacon Federal
Pro forma earnings per share from continuing operations:
Non-recurring expenses excluded from the unaudited pro forma supplemental information	6,700,000
Non-recurring gain on BFED stock excluded from the unaudited pro forma supplemental information	1,100,000
CBT
Pro forma earnings per share from continuing operations:
Non-recurring expenses excluded from the unaudited pro forma supplemental information	5,200,000	910,000
Greenpark
Pro forma earnings per share from continuing operations:
Non-recurring expenses excluded from the unaudited pro forma supplemental information	$ 555,000

Represents the amount of non-recurring expenses excluded from the pro forma information.

No definition available.

Name:	bhlb_BusinessAcquisitionProFormaInformationNonRecurringExpensesExcluded
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of non-recurring gain excluded from the pro forma information.

+ References

No definition available.

+ Details

Name:	bhlb_BusinessAcquisitionProFormaInformationNonRecurringGainExcluded
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	bhlb_BusinessAcquisitionsProFormaEarningPerShareFromContinuingOperationsAbstract
Namespace Prefix:	bhlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the pro forma interest income (expense), net for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

No definition available.

+ Details

Name:	bhlb_BusinessAcquisitionsProFormaInterestIncomeExpenseNet
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the pro forma non-interest income for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

No definition available.

+ Details

Name:	bhlb_BusinessAcquisitionsProFormaNonInterestIncome
Namespace Prefix:	bhlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(2)-(3)

-URI http://asc.fasb.org/extlink&oid=7659399&loc=d3e1392-128463

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 54

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 55

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 68

-Subparagraph r(2, 3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration